The mechanism by which the body prevents loss of blood from the vascular system is known as hemostasis. Blood maintains a state of fluidity in normal circulation, but forms a barrier when trauma or pathologic conditions cause vessel damage. Coagulation tests measure the blood's ability to form a clot or coagulate and are used to manage a patient's anticoagulant therapy and diagnose hemostatic disorders. Lysis tests measure the reverse change where one is measuring the lytic activity of coagulated blood which is broken down to soluble degradation products by, for example, an enzyme plasmin.
There are two well-recognized coagulation pathways: the extrinsic or thromboplastin-controlled and the intrinsic or prothrombin/ fibrinogen-controlled coagulation pathway. Both the extrinsic and intrinsic pathways result in the production of thrombin, a proteolytic enzyme which catalyzes the conversion of fibrinogen to fibrin. Two routine coagulation tests measure the Prothrombin Time (PT) and the Activated Partial Thromboplastin Time (APTT). Both tests measure clotting time to evaluate a patient's baseline hemostatic state or to monitor the response to anticoagulant therapy as well as the overall function and status of the coagulant system.
The PT test is used to assess the extrinsic and common pathway clotting systems and for monitoring long term anticoagulant therapy. A common medication for long term anticoagulant therapy is sodium warfarin isopropanol clathrate, generally known by the brand name COUMADIN.RTM., made by Dupont Pharmaceuticals of Wilmington, Del. Warfarin and its analogs induce anticoagulation by effectively blocking biosynthesis of Vitamin K dependent coagulation factors. Since the PT test measures clotting time, the effective amount of anticoagulant in the blood can be determined.
Another common medication which is in connection with cardiac bypass surgery, cardiac catheterization, renal dialysis, and in critical care situations for acute myocardial infarction is Heparin. The APTT test is widely used test for monitoring Heparin therapy for screening deficiencies of clotting factors included in the intrinsic and common coagulation system. Heparin exerts its anticoagulation effect by binding to and forming a complex with a plasma cofactor called antithrombin III.
Many laboratory clotting tests in the prior art are based on the phenomenon of measuring an endpoint which is a change of phase when a test sample changes from a liquid to a coagulated form. This phase change is due to the conversion of a soluble plasma protein fibrinogen to insoluble fibrin by the action of the enzyme thrombin. The clotting endpoint is physically detected by such secondary indicators as color or fluorescence detection by optical means and turbidity measurements through light scattering and magnetic particle oscillation. These laboratory instruments arc relatively large because of the complex technology used, expensive, and designed for use by trained personnel due to the complexity of the detection methods. See generally U.S. Pat. No. 5,344,754. Large blood samples are also usually required.
Some prior art devices use porous membrane supports impregnated with layers of a reagent for enzymatic assays which rely on monitoring the intensity of the reaction product by optical spectroscopy such as reflectance, fluorescence, luminescence or color change. Such reagent impregnated membranes increase the complexity of the reaction's environment due to the absence of a liquid phase which is the ideal environment for reactions or phase transitions, and could further lead to possible interference with the enzymatic pathway. The accuracy of the results from membrane based systems are further affected by blood hematocrit and reaction volumes. The need for larger liquid samples of blood to achieve complete wetability of the membrane imposes yet another constraint. To overcome some of the problems with membranes, U.S. Pat. No. 5,418,141 teaches the use of expensive, high purity reagents. However, coagulation techniques which use Thrombin substrate chemistries suffer from a major drawback due to their insensitivity to fibrinogen deficiencies which could yield inaccurate clotting times.
Many studies of blood coagulation have attempted to demonstrate that measuring blood resistance detects clotting time and obtains a quantitative measurement of the rate of clot retraction. The results were usually not reproducible and there was "considerable variation and inconsistency in most methods in common use" as disclosed in Rosenthal, R.L., and Tobias C.W.: Measurement of the Electrical Resistance of Human Blood; Use in Coagulation Studies, J Lab Clin Med 33, 1110, 1948. Critical emphasis was placed upon the geometric orientation of the cell within which a pool of the blood sample was retained and the electrodes. It was also critical to prevent vibration of the cell. It was observed that as soon as the blood clotted, clot retraction begins by the contraction of the fibrin network which pulls the large elements or cells together into a dense mass, thus displacing the serum to the periphery. This process produces increases in resistance measurements because it simultaneously increases the concentration of poorly conducting cells and decreases the concentration of serum, a good conductor, between and around the electrodes. Prior to clot formation there is no significant change in resistance. The clotting time and start of clot retraction are marked by the first increase in resistance. Thus the clotting time may be determined only with the elimination of motion. Subsequent increases in resistance resulted from retraction of the clot. The slope of the rising portion of the time-resistance curve was assumed to correspond to the rate of clot retraction.
As later disclosed in Hirsch FG, et al: The Electrical Conductivity of Blood I. Relationship to Erythrocyte Concentration, Blood 5: 1017, 1950, "Some workers have attributed these changes to the effects of coagulation, .sup.30-34 but others were unable to confirmn these findings. .sup.35,36 with certain designs of conductivity cells, blood resistance was observed to increase due to extrusion of serum during clot retraction, .sup.32,35. Blood conductivity was also found to vary with sedimentation, .sup.35,37,38 agitation, .sup.37 or stirring. .sup.39-42 " It is further reported in Table I on page 1018 that the conductivity was unchanged during clotting and that only during clot retraction that there was a decrease in conductivity.
As disclosed in U.S. Pat. No. 4,947,678, a device measures viscosity changes in blood to determine blood coagulation. An electrically conductive sensor heats a blood sample by passing current through the sample. The temperature of the sensor is averaged using its surface temperature and the current applied to the sensor. The sample temperature is also monitored and the difference between it and the average sensor temperature is calculated. Changes in the calculated temperature difference is used as the indication of viscosity change.
Thus, a need exists in the field of diagnostics for a method and device for measurement of blood coagulation or lysis which is sufficiently inexpensive, timely, efficient, convenient, durable, and reliable for use in a diagnostic device which permits point-of-care use by untrained individuals in locations such as the home, sites of medical emergencies, or locations other than a clinic. Whether the device is disposable or reusable, there is also a need to operate with small blood sample sizes.